Rudy Winston

Rudy Winston has over 16 years experience with Canon USA's Pro Products team, and has been responsible during that time for training Canon's staff on new products, creating presentations for customers and dealers, numerous writing projects, and providing technical assistance to professional and amateur photographers.

An inside look at Canon EF and tele extenders: Part II

For decades, tele extenders have been used by serious DSLR shooters and working professionals alike. Their appeal has never changed: an accessory that with modest size, weight and cost can increase the effective telephoto power of any lens it’s mounted to. Canon’s first EF Extenders, fully compatible with select L-series EF lenses, were launched in the late 1980s, within a year of the introduction of the first EOS 650 SLR camera. And ever since, they’ve been popular accessories for serious EOS shooters, from advanced amateurs to working professionals.

Over the years, Canon has offered three generations of EF 1.4x and 2x Extenders. In this report, we’ll clarify a few technical points about extenders in general and provide info about the different versions that have appeared through the years, and their compatibility with specific Canon EF lenses. In the Part I article, we discuss them from a photographic point of view and shed some light on how EF Extenders can really change the nature of different lenses in the EOS system.

Exposure with Canon EF Extenders

It’s a well-known fact that for physical reasons, a 1.4x Extender causes a light loss to the imaging sensor (or film) of one stop and a 2x Extender results in a two-stop light loss. In effect, this means that the lens you’re using becomes one or two stops “slower” with an extender attached. For example, an f/2.8 lens now has an effective maximum aperture of f/4 with a 1.4x Extender mounted, or f/5.6 with a 2x Extender. With older and all-mechanical film cameras, this loss of light meant that users had to mentally calculate the difference when setting apertures manually, if they were measured using a separate hand-held light meter. But the all-electronic lens mount of the EOS system was among the first in the industry to “do the math” for the photographer, converting aperture information so that the effective aperture is what’s always displayed in-camera. In other words, an f/2.8 lens with a 2x Extender would actually display f/5.6 in the viewfinder and on the camera’s LCD info panel as the widest possible aperture.

Exposure control, therefore, is pretty much seamless when a compatible EF lens is used with a Canon EF Extender. All exposure modes remain usable, from Manual to completely auto modes like Green Zone, Portrait mode and so on. All metering patterns remain available and the camera always accommodates the light loss in readings displayed by the metering system. EOS shooters never have to apply any compensation simply because an EF Extender is mounted. Even if using a separate hand-held meter, simply setting f-stops as indicated by the meter will result in proper exposure with no calculations or exposure compensation factors necessary.

EF lenses compatible with EF Extenders

Unfortunately, you can’t just attach a Canon EF 1.4x or 2x Extender to any Canon EF or EF-S lens. Canon has engineered some telephoto lenses to work properly with extenders, optically as well as electronically. One of the primary reasons you can’t simply mount an EF Extender to many lenses has to do with the optical design of the lens and extender. Many EF lenses (not designed to be extender-compatible) have rear lens elements that are nearly flush with the rear surface of the lens mount. If you look at a Canon 1.4x or 2x Extender, you’ll see its front element projects forward about ¼ of an inch (roughly 8mm or so). This is intentional, so that best optical performance can be achieved with those lenses that are officially extender-compatible. But with other lenses, this protrusion will physically prevent you from mounting the extender to the lens or at the very least restrict the zoom movement if you are able to physically attach the front of the extender to the rear of the lens.

The list of Canon EF lenses that are officially compatible with Canon’s EF Extenders includes the following:

The EF 100–400mm f/4.5-5.6L IS USM and the EF 200–400mm f/4L IS USM Extender 1.4x
(note that the EF 200–400mm f/4L IS USM Extender 1.4x can use EF 1.4x or 2x Extenders, in addition to its built-in 1.4x Extender)

All other Canon EF, EF-S, TS-E and MP-E lenses are officially not compatible with Canon’s EF Extenders and we do not recommend attaching them. Some photographers use accessories like an EF12 Extension Tube to allow physical attachment of an EF Extender to non-compatible lenses and in some cases, they get good images. Likewise, some shooters have used the same Extension Tube to permit “stacking” two or more tele extenders onto a lens. But please understand that Canon does not officially recommend or endorse the use of EF Extenders with non-compatible lenses and we cannot guarantee proper operation.

Three extra data contacts on lens mount for compatible EF lenses

If you look at the rear mount of a Canon EF lens that’s officially compatible with EF Extenders, you’ll see that it has three additional gold plated electronic contacts on its lens mount. But these are not present on lenses that are not extender-compatible. The extra contacts on the lens match with three additional contacts on the front mount of an EF 1.4x or 2x Extender, allowing a microcomputer in the extender to perform the following:

Convert lens focal length information being conveyed to the camera
This is done so that shutter speeds in Program AE mode can be shifted to faster settings, which is more appropriate for the effective increase in lens focal length. The converted focal length info is also recorded in a file’s metadata and can be viewed in applications such as Canon’s Digital Photo Professional software. This is the only time you’ll see any impact of this change in focal length information.

Converts reported lens aperture information to the camera body
With the extender in place, information about actual lens f-stops is converted to reflect the 1-stop or 2-stop light loss, in manual or automatic exposure modes. This changes the display seen in the viewfinder or the camera’s LCD information panel. This also has an impact on AF, as we’ll discuss briefly.

Reduces AF drive speed, to compensate for reduced focusing movement when extenders are in place

Change in lens AF speed with EF Extenders

This last point (directly above) has created sufficient misinformation that it warrants a separate explanation. With a 1.4x Extender attached, the amount of actual focus movement within a lens is reduced by 50%, in order to accommodate changes in actual subject distance. With a 2x Extender attached, the amount of movement of lens elements is reduced by 75%. Because AF systems are essentially computer-controlled to read and react to focus distance changes, the information must be modified so that the focusing movement (or sensitivity) compensates for the added presence of the extender. In the Canon EOS system, this is done by deliberately reducing drive speed when an extender is detected.

Before you immediately conclude that this is a problem, understand that this reduction in drive speed now corresponds to the effective speed you would achieve with the same EF lens alone. It compensates, automatically, for the reduced distance lens elements in the lens’s focusing group(s) need to move to refocus on a subject, with either EF Extender in place. Accordingly, overall AF performance remains essentially unchanged with an EF Extender attached, versus the lens’s AF speed without an extender.

This is another area of concern and, unfortunately, sometimes one of misinformation among serious DSLR shooters. With a few notable exceptions — we’ll discuss them below — all EOS DSLRs produced to date require that the effective maximum aperture of a lens or lens + extender combination be f/5.6 or faster to permit autofocus. With the automatic conversion of maximum aperture data performed via the additional electronic contacts on the extender’s front lens mount, if the camera sees an effective maximum aperture that’s slower than f/5.6 — f/6.3, f/7.1, f/8, or slower — it simply cuts off AF. (Note that all other functions, such as auto exposure and so on, remain fully active, even if AF is intentionally disabled by the camera).

This means that for AF to work, your lens itself must have a maximum aperture that falls within these parameters:

With EF 1.4x Extenders:
f/4 or faster (f/2.8, f/2, etc.)

With EF 2x Extenders:
f/2.8 or faster (f/2, etc.)

This does mean that lenses like the EF 500mm f/4L series, often used by nature and wildlife shooters, don’t support AF with the EF 2x Extender (with most EOS cameras). Similarly, the very popular EF 100–400mm f/4.5-5.6L IS USMis extender-compatible, but loses AF with most EOS bodies, even if the 1.4x Extender is attached.

An important note: this has nothing to do with the actual aperture you’re using to capture the image. The AF system is only concerned with the lens’s widest effective aperture for focus detection before a shot is taken. Whether you shoot the picture wide-open at f/4 or stop the lens down to f/22 for more depth-of-field or exposure control, it is totally irrelevant to AF compatibility. If the maximum aperture of your lens + extender combination is effectively f/5.6 or faster, you’ll get full AF — regardless of what shooting aperture, exposure mode and so on are used to take an actual shot.

It’s important to understand the basic reason for this. Again, contrary to some on-line speculation among photographers, it has nothing to do with the amount of light you’re shooting in. As a lens’s maximum aperture gets progressively smaller, the very precisely focused beams of light coming through the camera and onto its AF line sensors become physically smaller as well. This gets into the laws of physics, and cannot be changed.

AF sensors use pairs of rows of light-sensitive pixels during traditional phase-detection AF, which can instantly detect whether focus is correct or not. If it’s not, it can simultaneously and immediately determine what direction to move the lens’s focusing elements in and by how much. This is one reason why modern AF systems, when using normal viewing through the eye-level viewfinder, are usually so quick and positive without lots of back-and-forth lens movement.

However, as the focused “cone” of light becomes physically smaller, you reach a point where not all pixels on the AF sensor will be covered by the incoming light beams. Thus, the entire AF array is not available for focus calculation. This can and will happen, even in the brightest of shooting conditions — again, it’s related to the physical size of the focused beams of light coming to the AF sensor, not to the brightness or amount of that light. Canon’s engineers concluded that accurate and consistent AF cannot be assured with most EOS cameras when a lens + extender combination slower than f/5.6 is used with the vast majority of EOS models. Rather than risk a situation where AF may suddenly appear inconsistent, intermittent or inaccurate, it was safer to simply disable it altogether when this combination of equipment is detected.

AF down to f/8: EOS-1D X and EOS 5D Mark III

Canon’s recent high-end cameras with the 61-point AF system were initially introduced with the same limit of f/5.6 or faster for AF capability. But hearing the strong requests from nature, wildlife and other shooters who longed for the ability to have AF with an f/4 lens (such as the EF 500mm f/4L IS II USM or the EF 600mm f/4L IS II USM) along with a 2x Extender, modifications have been introduced via camera firmware upgrades that will permit AF with a lens + extender combination that’s f/8 or faster.

At a detected and effective maximum aperture of f/5.6 or faster, all AF points can be used for AF with these cameras. If the lens + extender combination shows an effective widest aperture that’s slower than f/5.6, down to f/8, AF now is possible using either:

The centermost AF point, alone
(cross-type coverage, an important addition, does remain active at this AF point; Spot AF is not possible)

AF Area Expansion, with the center AF point as the primary one, and four surrounding AF points on “stand-by”
(cross-type AF coverage, again, with the centermost AF point and single-line sensor coverage with the surrounding four points)

Previous generation top-of-the-line EOS SLRs with the 45-point AF system, for technical reasons, are able to use the center AF point only for AF when a lens slower than f/5.6, down to f/8, is detected. At f/5.6 or faster effective maximum aperture, all AF points can be used for AF. Once the detected maximum aperture is slower than f/5.6, the system switches AF to the center AF point exclusively. Unlike in the latest 61-point AF system, these cameras always use a single-line sensor at the center point for AF detection, not cross-type AF coverage.

AF is thus possible at f/8, center point only, with all versions of the EOS-1D and EOS-1Ds (up to Mark IV models), as well as the 35mm film-based EOS-3 and EOS-1v cameras.

Different versions of Canon’s EF Extenders

Since their introduction in the late 1980s, Canon has marketed three generations of 1.4x and 2x EF Extenders for the EOS system to date. Generally, compatibility with various EF lenses and EOS cameras has remained consistent. All EF extenders can be used with all extender-compatible EF lenses. Even if a user has, for example, the latest EF 500mm f/4L IS II super-tele lens, an original EF 1.4x Extender can indeed continue to be used with that lens However, there is an important restriction to be aware of with the newest Version III extenders with older EOS film-based SLRs.

Here is a quick run-down of the basic generational differences among Canon extenders:

Original EF 1.4x and EF 2x (introduced November 1988 and October 1987, respectively)

There’s no doubt that Canon’s EF Extenders continue to be an important part of many telephoto lens users’ equipment arsenal. But users who, perhaps, have resisted their call up to now may reconsider, especially in the light of the recent firmware updates for the EOS-1D X and EOS 5D Mark III cameras. In this article, we’ve attempted to clarify many of the operational features (and limitations) that an EF Extender can impose, as well as any lingering questions about camera or lens compatibility. With an understanding of how Canon’s extenders work and why, it will hopefully be easier for EOS users to decide whether EF Extenders are the right answer for their needs and, if so, whether one (or both!) are appropriate to include in their camera bags.

The CDLC contributors are compensated spokespersons and actual users of the Canon products that they promote.